Problem: Which of these numbers is prime? ${11,\ 15,\ 21,\ 27,\ 56}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 11 are 1 and 11. The factors of 15 are 1, 3, 5, and 15. The factors of 21 are 1, 3, 7, and 21. The factors of 27 are 1, 3, 9, and 27. The factors of 56 are 1, 2, 4, 7, 8, 14, 28, and 56. Thus, 11 is a prime number.